In addition to the single-cell microbes that abound in nature, there are also various types of multicellular forms; these have more complex internal structures and reproduce in more complicated ways than the single-cell microbes. Yeasts, microalgae, and protozoa are types of single-cell eukaryotes that differ from one another in various ways, for example, in how they obtain energy for growth. Opposite are the prokaryotes, that is, bacteria. These organisms have a comparatively simple anatomy and do not have a distinct nucleus. The DNA of bacteria is the same kind found in other living organisms, but it is organized in a different fashion, rather like a fuzzy blob floating free in the cell interior. The prokaryotes are unicellular organisms and include the taxonomic domains Eubacteria and Archaebacteria. The term Archaebacteria implies that this domain includes organisms of great antiquity, but there is no evidence to support this notion. The Eukaryotes include all the rest of life on Earth except the Bacteria and Archaea. Among them are grouped the yeasts, fungi whose usual growth mode is unicellular. Fungi are multicellular eukaryotic microbes of great importance with respect to chemical change in the biosphere, infectious diseases of plants and animals, and production of antibiotics. Much of the contemporary molecular biology widely discussed in newspapers and news magazines is based on applying knowledge derived from the study of bacteria to the analysis of processes in plants and animals, including humans.

Typical structures of prokaryotic and eukaryotic cells. (Top) Typical bacteria are quite small, on the order of a few micrometers (1 micrometer = 1-millionth of a meter). Some can contain granules of storage material (usually carbohydrate or fat) that provide carbon and energy for growth under certain conditions. Motile species also have thin, whiplike appendages (flagella) used for locomotion; others have shorter surface appendages known as fimbriae or fibrilla. (Bottom) Single-celled eukaryotes are generally much larger than bacteria. This diagram illustrates the structural features of a typical eukaryotic green algae cell. Within the cell are several kinds of distinct “organelles”: N, the nucleus, containing chromosomes; CP, chloroplasts, which contain chlrorophyll, the green pigment that enables the cell to capture and use light as a source of growth energy (photosynthesis; Chapter 15); M, mitochondria, which are present in all algal, plant, and animal cells. In algae and plants, mitochondria catalyze aerobic respiration (see Chapter 9), which provides energy for cellular processes when the organisms are in darkness.

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Figure 3

Typical structures of prokaryotic and eukaryotic cells. (Top) Typical bacteria are quite small, on the order of a few micrometers (1 micrometer = 1-millionth of a meter). Some can contain granules of storage material (usually carbohydrate or fat) that provide carbon and energy for growth under certain conditions. Motile species also have thin, whiplike appendages (flagella) used for locomotion; others have shorter surface appendages known as fimbriae or fibrilla. (Bottom) Single-celled eukaryotes are generally much larger than bacteria. This diagram illustrates the structural features of a typical eukaryotic green algae cell. Within the cell are several kinds of distinct “organelles”: N, the nucleus, containing chromosomes; CP, chloroplasts, which contain chlrorophyll, the green pigment that enables the cell to capture and use light as a source of growth energy (photosynthesis; Chapter 15); M, mitochondria, which are present in all algal, plant, and animal cells. In algae and plants, mitochondria catalyze aerobic respiration (see Chapter 9), which provides energy for cellular processes when the organisms are in darkness.